A lot comes into play regarding MPG when towing... One of the determining factors is the turbulence created just behind a tow vehicle. Sometimes the flat front trailers actually do a better job of breaking up the drag caused by turbulence from the tow vehicle.
All that being written, I've always believed that if I had to worry about the money I spend camping then I shouldn't be camping. The relaxation, quality family/friend time and sheer enjoyment of the outdoors far outweighs any cost.
My philosophy is to buy the biggest trailer you can afford and your truck has the capacity to tow and enjoy. I would hate to be stuck in a small trailer during a couple of days of bad weather...cabin fever kicks in pretty quick. The more room the better.